This shopping thing is just so petty.
What's the rule that says you have to automatically become best friends with your husbands partner?
DH is an only child so I've no direct comparison, but in a similar vein the wife of his best friend and I have nothing in common.
Don't get me wrong - if we are out in a group we chat cordially and there's no animosity, but in terms of personalities, interests, political leanings we are chalk and cheese. It wouldn't occur to either of us that we would arrange to spend time shopping (or anything else) solely in each other's company.
Also it just doesn't ring true given how gushy they were about Catherine in the engagement interview.
It all just smacks again of expecting the world and everyone in it to run around them, that they have to be front of stage in not just their own lives but everyone else's and in every capacity, all the time.
I've no idea why Catherine didn't ask Meghan shopping - she might have been in a rush, she didn't know Meghan wanted to go, she didn't want the additional publicity of them being photographed together or quite simply might have wanted an hour to herself.
You don't get to impose yourself on someone else's life.